<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        @keyframes ants { to { background-position: 10% } }
        .marching-ants { padding: 1em;
            border: 1px solid transparent; background:
                linear-gradient(white, white) padding-box,
                repeating-linear-gradient(-45deg,
                black 0, black 25%, white 0, white 50%
                ) 0 / .6em .6em;
            animation: ants 12s linear infinite;
        }
    </style>
</head>
<body>
<div class="marching-ants"></div>
</body>
</html>